Field of the Invention
The present invention relates to a frequency control apparatus using a PDM system and a motor driving apparatus configured to drive a vibration motor using the same.
Description of the Related Art
One of the pulse modulation systems is a Pulse Frequency Modulation (PFM) system configured to modulate a frequency of a carrier period. The PFM system is classified into a system configured to steplessly change a frequency, and a system configured to simulatively change a frequency by thinning out a frequency clock to be used. The illustrative former system is a Voltage Controlled Oscillator (VCO) system that can obtain a comparatively fine frequency resolution for an input voltage. The illustrative latter system is a Pulse Density Modulation (PDM) system that increases the frequency resolution by changing an average frequency per a unit time period while maintaining constant the input clock. The PDM system that can be implemented by a digital circuit is economically more advantageous than the VCO system that can be implemented by an analogue circuit.
Since the PDM system obtains a high frequency resolution by changing the frequency for each predetermined time period and by averaging the result along the time axis, the output frequency does not become constant and scatters when it is observed at predetermined time intervals. Therefore, in controlling the driving system to which an output signal generated by the PDM method (PDM control) is input, the frequency scattering in the PDM control causes an uneven speed profile for the driving system and a signal in an audible band.
Japanese Patent No. 4838567 discloses a motor driving apparatus that makes high a resolution of the driving frequency applied to the vibration motor by the PDM control using an adder, and controls a motor by adding random noises to the frequency scattering by the PDM control using the adder. The motor control apparatus adds the random noises, disperses the frequency, and reduces a frequency component having a high spectrum for a specific setting oscillatory frequency and thereby a peaky frequency spectrum in the audible band.
However, the motor driving apparatus disclosed in Japanese Patent No. 4838567 simply disperses the spectrum intensity by adding the random noises to the periodical frequency component different from the driving frequency. This motor driving apparatus can disperse the frequency spectrum intensity and prevent the frequency component having a high spectrum intensity in the audible band, but the signal in the audible band itself remains. Hence, the signal in the audible band (noise) may be amplified by the resonance in the motor installed apparatus, etc. In addition, the frequency scattering may increase at predetermined time intervals due to the random noises, and the uneven speed of the driving system may increase.